This invention relates to a reflection-type photoelectric switching apparatus which includes a light projecting element for projecting a beam of light to an object, a light receiving element for receiving a reflected light beam from the object and a circuit unit for processing output signals generated from the light receiving element, and more particularly to an improved photoelectric switching apparatus capable of generating a steady switching output signal regardless of any change in the environment surrounding the light receiving element.
The conventional reflection-type photoelectric switching apparatus is essentially such that the presence of an object is detected by comparing a predetermined reference signal level with an output signal amplitude obtained from a light receiving element which receives reflected light rays from the object. The reference signal level can be determined by colors or reflectances of the object and matter adjacent to the object. When the application process is that of cutting a sheet of paper to a predetermined size, for instance, the reference level is determined in accordance with the color of the sheet and of markings, thus defining the predetermined size of the sheet detected by the switching apparatus.
Generally, an LED (light emitting diode) or an incandescent lamp is utilized as a light source for a light projecting element comprised in the switching apparatus. Such a light projecting element has a given wavelength characteristic, so that the reflected light incident upon the light receiving element also has a similar spectral wavelength characteristic. The distribution of energy emitted from the light projecting element is, however, apt to vary with aging. Moreover, a range of wavelengths in which the light receiving element is sensitive to the reflected light is liable to vary with a change of temperature applied to the light receiving element. Thus, the wavelength characteristic of the output signal amplitude obtained from the light receiving element is unstable, because it is liable to vary with changes in the environment surrounding the light receiving element.